The tumor suppressor protein p53 is a transcription factor that has been implicated in the cellular response to DNA damage and mediates either growth arrest or apoptosis. The tumor suppressor BRCA1 is less well- characterized but over-expression of BRCA1 was shown to also alternatively lead to growth arrest or apoptosis p53 and BRCA1 have been shown to interact both physically and functionally. The research proposed in the application is designed to explore the details of p53 and BRCA1-dependent transcriptional activation and elucidate whether the interaction of these tumor suppressor proteins has biological relevance. Preliminary studies have identified two classes of p53 response elements by examining the effect of monoclonal antibody moAb421 on the sequence-specific DNA binding of p53 in electrophoretic mobility shift assays. Incubation with moAb421 enhances the binding of p53 to one set of response elements by inhibits binding to another set, suggesting that different conformations of p53 may interact with each subset of response elements. BRCA1 selectively enhances p53-dependent activation of one subset of sites. Our studies have also shown that the ATF1 transcription factor can interact with BRCA1 protein both physically and functionally and that BRCA1 can augment transcription through a cyclic AMP response element (CRE). Promoters have been identified which contain both a CRE and a p53 binding site. Thus, the proposed aims include experiments to: (1) Determine the significance of distinct subsets of p53 response elements classified by the differential effect of monoclonal antibody moAb421. (2) Determine how BRCA1 alters transcription through the CRE and characterize the interaction of BRCA1 with members of the ATF1 and CREB transcription factors. (3) Characterize the interaction of BRCA1 and p53 on genes that contain both a CRE and a p53 site. (4) Determine the role of BRCA1 and ATF1 in the p53- mediated response to DNA damage. As p53 is mutated in up to 30% of cases of breast cancer and the BRCA1 gene is responsible for approximately 50% of inherited breast cancer, it will be important to determine whether these two tumor suppressor proteins play convergent roles in gene expression involved in the cellular response to DNA damage as well as in the pathogenesis of breast cancer.